1. Field of the Invention
This invention relates to methods and apparatus for controlling sharpness of images in a printing plate making process and the like. More particularly, the invention relates to a technique of controlling, with unsharp masks, sharpness of color images separated into red (R), green (G) and blue (B), or the like.
2. Description of the Related Art
As is well known, unsharp mask processing is very effective and widely practiced as means of controlling image sharpness to improve the visual impression of reproduced images. The principle of unsharp mask processing will be described hereunder, taking a digitized monochromatic image for example.
Unsharp mask processing is performed through the following steps (A)-(C):
(A) First, a so-called blurred image (unsharp mask) is prepared from an original image. The blurred image is obtained by repeating an operation, for the entire original image, to replace image data of a key pixel with averaged image data of neighboring pixels. PA1 (B) A difference between the original image and blurred image is obtained for each pixel, and the difference is regarded as an edge component of the pixel. PA1 (C) The edge component is added to the corresponding pixel. The degree of sharpness may be controlled by multiplying the edge component by an appropriate numerical value prior to adding the edge component. PA1 (A) A gray image is prepared by extracting gray components from an original image. This gray image is used to prepare a blurred image of gray components. PA1 (B) A difference between the gray image and the blurred image of gray components is obtained for each pixel, and the difference is regarded as an edge component of the pixel. PA1 (C) The edge component of each pixel is added equally to the RGB components of each pixel in the original image. PA1 (A) Blurred RGB images are prepared from the planes of RGB components in the original image, respectively. PA1 (B) A difference between the plane of R component in the original image and the blurred image of R component is obtained for each pixel, and the difference is regarded as an R edge component of the pixel. G edge component and B edge component are determined likewise. PA1 (C) The R edge component of each pixel is added to the R component of each pixel in the original image. Similarly, the G edge component is added to the G component, and the B edge component to the B component. PA1 a first edge component generating step for preparing a gray image of the original image by extracting gray components from the original image, preparing a gray unsharp mask which is a blurred gray image from the gray image, and determining a difference between the gray image and the gray unsharp mask for each pixel, the difference being regarded as a gray edge component of each pixel; PA1 a second edge component generating step for preparing color unsharp masks which are blurred images from respective color components of the original image, and determining a difference between an image composed of components of each color and a corresponding one of the color unsharp masks for each pixel, the difference being regarded as a color edge component of each pixel; PA1 a mixing ratio acquiring step for acquiring a mixing ratio between the gray edge component obtained at the first edge component generating step and each color edge component obtained at the second edge component generating step; PA1 an edge component mixing step for performing an operation, for each pixel and for each color component, to mix, in the mixing ratio acquired, the gray edge component generated at the first edge component generating step and each color edge component generated at the second edge component generating step; and PA1 an adding step for performing an operation, for each pixel and for each color component, to add each mixed color edge component obtained at the edge component mixing step to a corresponding one of the color components of the original image. PA1 a first edge component generating device for preparing a gray image of the original image by extracting gray components from the original image, preparing a gray unsharp mask which is a blurred gray image from the gray image, and determining a difference between the gray image and the gray unsharp mask for each pixel, the difference being regarded as a gray edge component of each pixel; PA1 a second edge component generating device for preparing color unsharp masks which are blurred images from respective color components of the original image, and determining a difference between an image composed of components of each color and a corresponding one of the color unsharp masks for each pixel, the difference being regarded as a color edge component of each pixel; PA1 a mixing ratio determining device for evaluating saturation of each pixel in the original image successively and determining a mixing ratio of the edge components for each pixel based on the saturation, such that, for pixels having low saturation, the edge component generated by the first edge component generating device is mixed in a larger amount than each edge component generated by the second edge component generating device, and for pixels having high saturation, each edge component generated by the second edge component generating device is mixed in a larger amount than the edge component generated by the first edge component generating device or in the larger amount for the lower saturation; PA1 an edge component mixing device for performing an operation, for each pixel and for each color component, to mix, in the mixing ratio determined by the mixing ratio determining device, the gray edge component generated by the first edge component generating device and each color edge component generated by the second edge component generating device; and PA1 an adding device for performing an operation, for each pixel and for each color component, to add each mixed color edge component obtained by the edge component mixing device to a corresponding one of the color components of the original image. PA1 a first edge component generating device for preparing a gray image of the original image by extracting gray components from the original image, preparing a gray unsharp mask which is a blurred gray image from the gray image, and determining a difference between the gray image and the gray unsharp mask for each pixel, the difference being regarded as a gray edge component of each pixel; PA1 a second edge component generating device for preparing color unsharp masks which are blurred images from respective color components of the original image, and determining a difference between an image composed of components of each color and a corresponding one of the color unsharp masks for each pixel, the difference being regarded as a color edge component of each pixel; PA1 a mixing ratio designating device for designating a common fixed mixing ratio between the edge component obtained by the first edge component generating device and each edge component obtained by the second edge component generating device; PA1 an edge component mixing device for performing an operation, for each pixel and for each color component, to mix, in the mixing ratio designated by the mixing ratio designating device, the gray edge component generated by the first edge component generating device and each color edge component generated by the second edge component generating device; and PA1 an adding device for performing an operation, for each pixel and for each color component, to add each mixed color edge component obtained by the edge component mixing device to a corresponding one of the color components of the original image.
A similar process is performed for a color image, but the following two methods are used in relation to formation of edge components:
First Method!
It is assumed here that a color image is separated into RGB components which are digitized.
A sharpened color image results from the above steps (A)-(C).
Second Method!
Similarly, it is assumed that a color image is separated into RGB components which are digitized.
A sharpened color image results from the above steps (A)-(C).
Conventional sharpness processing apparatus employ the first method or the second method noted above, but either causes the following inconvenience:
In an apparatus employing the first method, pixels having high degrees of saturation in the original color image fade in color to become white or black (hereinafter referred to also as "white edges or black edges"), as a result of unsharp mask processing.
In an apparatus employing the second method, achromatic or nearly achromatic pixels in the original color image assume strong color (hereinafter referred to also as "colored edges"), as a result of unsharp mask processing.
The reasons why the colored edges and the white or black edges occur will be described hereunder.
Reason for Appearance of White or Black Edges!
Reference is made to FIG. 1. Assume that a row of pixels on one scan line in an original image includes gray and red lying adjacent each other as shown in FIG. 1(a). The RGB components forming this portion of the original image are shown in FIG. 1(b), (c) and (d).
The following description is based on the assumption that unsharp mask processing in the first method is effected for this portion.
FIG. 1(e) shows a gray component (gray image) extracted from the original image shown in FIG. 1(a). A blurred image (unsharp signal) of this gray component is as shown in FIG. 1(f). A gray edge component as shown in FIG. 1(g) is obtained by subtracting the blurred image (FIG. 1(f) of the gray component from the gray image (FIG. 1(e)). When this edge component is added equally to the RGB components (FIG. 1(b), (c) and (d)) of each pixel in the original image, sharpened RGB components as shown in FIG. 1(h), (i) and (j) are obtained.
As seen from FIG. 1(h), (i) and (j), gray pixels at the boundary between gray and red are darkened (with the levels of RGB components lowered equally) as a result of sharpness processing. This gives no visual incongruity. However, with the red pixels at the boundary, the gray edge component added to the RGB components raises the levels of the respective components. As a result, the red pixels at the boundary become white to form a white edge. This causes visual incongruity because of a white streak running through the originally red area. Even if a distinct white or black streak is invisible, the stronger unsharp mask processing tends to make the image the less bright. Particularly where the original image includes thin, bright-color lines, color information of the lines will be lost, thereby to lower image quality markedly.
Reason for Appearance of Colored Edge!
Reference is made to FIG. 2. As in the foregoing description, it is assumed that unsharp mask processing in the second method is effected for an area where gray and red are present adjacent each other as shown in FIG. 2(a). FIG. 2 (b), (c) and (d) show RGB components corresponding to FIG. 2 (a).
FIG. 2(e), (f) and (g) show blurred images (unsharp signals) of the RGB components shown in FIG. 2(b), (c) and (d), respectively. RGB edge components as shown in FIG. 2(h), (i) and (j) are obtained by subtracting the blurred images shown in FIG. 2(e), (f) and (g) from the RGB components shown in FIG. 2(b), (c) and (d), respectively. When these edge components are added individually to the RGB components (FIG. 2(b), (c) and (d)) of each pixel in the original image, sharpened RGB components as shown in FIG. 2(k), (l) and (m) are obtained.
As seen from FIG. 2(k), (l) and (m), gray pixels at the boundary between gray and red become blue or green, with the R component decreased (see FIG. 2(k)) and the GB components increased (see FIG. 2(l) and (m)) as a result of sharpness processing. This forms a colored streak in the gray side of the boundary to give visual incongruity. With the red pixels at the boundary, the R component is added and the GB components decrease. Since the red area is only accentuated to be redder, no visual incongruity is caused.